Deformable nozzle structures



Jan. 25, 1966 Filed Jan. 2, 1963 DfFZfU/ON FIG-2.

E. F. NEEDHAM ETAL 3,231,037

DEFORMABLE NOZZLE STRUCTURES 3 Sheets-Sheet l INVENTOKS E LF. NEEDHAM R.B. PAGE 623%, Mm M 1966 E. F. NEEDHAM ETAL 3,231,037

DEFORMABLE NOZZLE STRUCTURES Filed Jan. 2, 1963 3 Sheets-Sheet 2 1N VEN T026 EJE. NEEDHAM R. B. PAGE ATTORNEYS Jan. 25, 1966 E. F. NEEDHAM ETAL 3,231,037

DEFORMABLE NOZZLE STRUCTURES Filed Jan. 2, 1963 3 Sheets-Sheet 5 .50 5/ 4455/47 50 59K z w mvzwroes BF. NEEDHAM RJES. PAGE,

A TTOZA EYS W nd. State I Pat n 3,231,037 DEFORMABLE NOZZLE STRUCTURES Ernest Frederick Needhain, HighclitEe Ham shire, and

Reginald Bannerman Page, Hythe, Hampshire, England, assignors to Hovercraft Development Limited,

London, England, a British company Filed Jan. 2, 1 9'63, Ser'. No. 249,031 Claims priority, application Great Britain, Jan. 4, 1962, 429/ 62 X 11 Claims. (c1. 180-7) The invention relates to deformable structures, that is to say to structures which in their normal shape serve by virtue of their shape a particular function but are adapted in certain circumstances to be deformed temporarily, recovering their normal shape when the circumstances causing the deformation cease.

Such structures find application by Way of example in vehicles of the kind adapted to be supported on a cushion of pressurised gas maintained beneath the vehicle by means of one or more streams of fluid directed into the space beneath the vehicle through suitably located and suitably shaped nozzles located in the base of the vehicle. Becauseof their location the nozzles through which the fluid is directed may be exposed to mechanical forces due for example to impact with obstacles in the path of the vehicle which could damage the nozzles or at least retard the progress of the vehicle and it is of advantage in some circumstances to make the nozzles flexible and resilient so that they will yield to these forces without being damaged and will recover their proper shape when relieved from the distorting forces. The problem therefore is to provide a form of structure which will retain a shape suitable for the function of directing the fluid stream as required during all normal conditions but which will yield, for example on impact with an obstruction in the path of the vehicle, to allow the vehicle to ride over the obstruc tion without undue resistance to its passage. The obstruction might, for example, be a Wave, if the vehicle is travelling over water, or a rock or boulder if over land.

According to this invention in one aspect, a structure required to perform a given function by virtue of its shape, but temporarily deformable by mechanical action, is formed as an inflatable flexible structure constrained to assume when inflated the configuration appropriate to its function, the space within said structure which receives the inflating gas being in free communication with a recuperation region the arrangement being such that changes in inflation pressure within the structure due to deformation of the structure by external mechanical forces are small. The effect may he achieved, for example, by allowing the flexible inflated structure to communicate" with a substantially fixed volume recuperation region which may be an enclosed chamber of rigid construction or it may be in communication with a pressuri'sing source. In either event the communication should be through low impedance ducts or apertures so as to reduce to an acceptable degree temporary increases of pressure within the structure which might arise due to impulsive deformations. V H I In one particular form of the invention there is provided an inflatable structure constrained to assume a predetermined shape when inflated comprising at least two flexible walls, constraining means connecting saidwalls, and means for supplying a gas under pressure to inflate the structure, the space within said structure being in communication with a recuperation region in such a manner that changes in inflation pressure within the structuredue to deformation of the structure are reduced to a desired degree, in relation to the changes which would occur without said recuperation region.

ICC

In a further form of the invention there is provided a flexible nozzle having two walls each comprising an infiatable structure constrained to assume, when inflated, a predetermined shape, havng at least two flexible walls, constraining means connecting said flexible walls, and means for supplying a gas under pressure to inflate each structure, the space within each structure being in communication with a recuperation region such that changes in inflation pressures within the structures due to deformation of the structures are small.

Structures according to the invention may be maintained inflated by continuous exposure to a source of inflation pressure, and in the case of the suggested application to the nozzles of gas-'cushion-supported vehicles above referred to, the source may be the same source as that which supplies the gas to the vehicle-supporting cu'shion. In such a case the further advantage may be achieved that the stiffness of the inflated structure, which is of course related to the pressure of the inflating gas, is variable in conformity with the pressure of the gas in the vehicles'upporting cushion or the gas stream or streams supplying it. The deformability of the nozzles is thus increased as the power supplied to the fluid curtain is decreased.

The nozzles and their associated structure are subject to various loads and it is desired that the structure should retain, under normal conditions, a certain designed configuration. By connecting the inflated structure to the same source as provides the gas for the cushions, the stiffness of the structure is maintained, to a large extent, in line with the loads, and with the variation of these loads, and the configuration of the inflated structure will be maintained substantially as desired. v

Inflatable structures of which the shape when inflated is stabilised so as to fulfil a function by virtue of their shape are of course known, but in contrast to known structures of this kind the structures according to this invention are required to combine the properties of stilfness in all normal conditions and deformability in response to certain conditions. The known structures are either of one kind in which stiffness is not achieved and is not necessary at any time or of another kind in which stiffness is achieved at all times. It has been found that by the provisions of the present invention of the two properties can be achieved in the same structure, and the following explanation given with reference to FIGURE 1 of the accompanying drawings will show how the characteristics ofthe structure can be adjusted to the requirements of the purpose to which it is applied.

FIGURE 1 is a family of curves (a), (b) and (c) showing how a structure according to the invention deforms as the force causing the deformation increases. The three curves illustrate the effect of difference inflation pressure, curve (a) being the lowest inflation pressure, curve (1;) a slightly higher pressure and curve (c) a higher pressure still. Over the initial part of each curve displacement increases substantially linearly with force. At a given level, however, there is a knee in each curve and beyond this point the slope of each curve is very much less steep. The point at which the knee of the curve occurs is a function mainly of the pressure within the system and the slope of the curve beyond the knee is primarily a function of the inherent stiffness of the structure, and depends upon the properties of the mate'- rials of which it is made. If the structure were of negligible intrinsic stiffness it would collapse immediately the force exceeded the val ue at the knee of the curve.

The curve of FIGURE 1 is an idealised example in the sense that it assumes a constant pressure, low impedance system. If the inflating medium works into a recuperation region which includes the pressurising source which maintains constant pressure within the system then the linearity of the linear portion of the curve below the knee will be good. If, however, the recuperation region is a constant volume chamber the linearity of the curve below the knee will depend upon therelative volumes of the inflated, deformable portion of the structure and the fixed volume of the recuperation region. Obviously, as the volume of the recuperation region in a fixed volume system decreases towards zero the characteristic of the deforma'ble structure approaches that of a simple pneumatic spring, the knee becomes less pronounced and the curve becomes smoother.

The characteristics of the system may also be modified by making some parts thereof of more readily extensible material.

Some examples of structures illustrating the invention will now be described with reference to FIGURES 2-7 of the accompanying drawings in which:

FIGURE 2 is a cross-section through a nozzle structure for an air cushion supported vehicle employing a simple air curtain for retaining the cushion,

FIGURE 3 is a cross-sectional view of a more complicat-ed structure embodying a plurality of nozzle parts constructed in accordance with the invention, applicable to an air cushion supported vehicle employing a retaining air curtain with a recirculating air system,

FIGURE 4 is a section on the line AA of FIGURE 3,

FIGURE is a detail showing a method of construction which may be employed in structures according to the invention,

' FIGURE 6 is a cross-section on the line B-B of FIG- URE 5, and

FIGURE 7 is a vertical cross-section of a vehicle embodying one form of the invention.

Referring first to FIGURE 2, this illustrates in crosssection a nozzle of an air cushion supported vehicle of the kind described in U.S. patent application Ser. No. 627,925 in which the pressurised air cushion is retained beneath the vehicle at least in part by a stream of pressurised air directed inwardly from the periphery of the craft. The nozzle comprises two structures 1 and 2 each of which is according to the present invention. Air is supplied to the nozzle for the purpose of forming the air curtain through a duct 3 which is formed by two hollow members 4 and 5. The structure 1, which constitutes the outer part of the nozzle, extends in a substantially semicircular arc from the upper hollow member 4. The structure 2, which constitutes the inner part of the nozzle, is of a wedge-shaped cross-section and extends from ,the lower hollow member 5. The two structures 1 and 2 define between them a tapering nozzle directed inwardly with respect to the vertical at a mean angle of about 45. The hollow members 4 and 5 which form the main supply duct communicate with the interiors of the structures 1 and 2 through orifices 6 and 7 which are made as large as is practicable so as to reduce to a minimum resistance to the flow of air between the structures and the members 4 and 5.

Each of the structures 1 and 2 has walls of flexible airimpervious rubber-like material such as neoprene sheeting reinforced with fabric and the cross-sectional shape of each structure is stabilised against dilation when inflated by means of cords 8 and 9 which extend between the two walls of each structure substantially normal to the central axis of the structure when it is in its normal inflated shape. The curved surface 10 of the structure'Z is also stabilised against dilation by cords 11, although the amount of stabilisation provided can vary depending upon the shape of this part of the structure wall and on the degree of dilation which can be tolerated. The cords 8, 9 and 11 are effective only in tension and resist dilation of the structure by the inflating air; when the structure is required to collapse, however, these cords offer no resist ance.

The structure shown maybe positioned at the front or theback of the vehicle. If the vehicle is intended to op- V 4 erate over water then the flexible nozzle shown will be required to deflect in either direction on impact with waves so as to allow the vehicle to ride over the waves without undue resistance to its passage.

The outer structure 1 is attached to the upper hollow member 4 by means of headings 12 let into the seating 13 supported in the end of the member. Similarly the inner structure 2 is attached to the end of the hollow member 5 by means of beadings 14- set into grooves on a seating 15 supported in the end of the member 5.

Air is supplied to the interiors of the two structures 1 and 2 through the hollow members 4 and 5 at a pressure suflicient to inflate the structures to a rigidity adequate to sustain them in their proper configuration against the normal pressure of the air supplied through the main duct 3, against the cushion pressure and also against the normal small resistance to forward motion of the vehicle through the air; plus perhaps a small margin for light contacts with the surface. Assuming that the nozzle is mounted at the rear of a vehicle, that is, the movement of the nozzle is normally from left to right in FIGURE 2, when the nozzle structure encounters a heavy resistance, for example, if the vehicle rides into a wave as above described, the nozzle is deflected to the left as seen in the drawing and assuming that the loading thereby exerted on the structure is in excess of the knee load the hollow walls of the structure will collapse, expelling the inflating air from within them against the pressure in the system supplied through the hollow members 4 and 5. If the nozzle meets with an obstruction moving in the reverse direction, for example if the vehicle moves backwards, then the nozzle will deflect towards the right in FIGURE 2.

Similarly, if the nozzle is mounted at the front of a vehicle, that is, the movement of the nozzle is normally from right to left in FIGURE 2, the nozzle will deflect to the right as seen in the drawing when meeting an obstruction, whilst if the vehicle moves backwards, for example, and the nozzle meets an obstruction, it will deflect to the left as seen in the drawing.

Assuming that the members 4 and 5, as will normally be the case, communicate directly with a source of pressurising air which is continuously in operation, the resistance to expulsion of air from the structures will be sensibly constant and determined by the impedance of the pressurising system. When the impact ceases, the pressure within the members 4 and 5 is available to reinflate the structures and erect them to their normal configuration, in which shape they are stabilised by the cords 8, 9 and 11. Apart from the exceptional circumstances of an impact such as has just been described the structure retains its shape against all the normal deflecting forces it is liable to encounter. In terms of the description given above which reference to FIGURE 1, normal operation of the vehicle involves deflection forces upon the nozzle structure which produce deflections which are on the linear part of the curve. Only when there is impact with an obstacle do the forces rise to a value large enough to carry the structure beyond the knee of the curve into the region in which it collapses.

Referring now to FIGURES 3 and 4, this shows a more complicated ducting system intended for an air cush on vehicle in which the air cushion is retained by an air curtain of more complex form in which at least part of the air is recovered and reused. The system in-- cludes three nozzles 20, 21 and 22, the centre one 21 of which is fed with energised air constituting the main air supply to the cushion retaining curtain via duct 23. At least part of the air forming the air curtain is recovered into nozzle 22. From the nozzle 22 the recovered air flows through a duct 24 to nozzle 20 from which issues a stream of air constituting an outer curtain. The duct 23 and duct 24 subdivide so that they can be interleaved to allow flow of air to the respective nozzles 21 and 20. The mouth parts of the nozzles 20, 21 and 22 are all made the inflated structures and the pressurising source.

5 as flexible structures according to this invention. Four such structures are shown, 26, 27, 28 and 29. The part 26 is of tapered cross-section and extends downwardly and inwardly from a duct or chamber 30 which supplies it with inflating air pressure. Part 27 is similarly mounted .on the mouth of a duct or chamber 31 and is again a simple tapered structure. The part 28, mounted on and inflated through a duct or chamber 32, is of more cornpl'icated configuration since it includes a re-entrant flute :33 which is required for the purpose of correctly organising the .air flow into a vortex which circulates between the nozzles 21 and 22. Part 29 is again a simple tapered structure mounted on and inflated through a duct .or chamber 34. Each of the parts 26, 27, 28 and 29 can be attached to its respective duct by means of beadings and 'seatings as the previous example. Part 26 illustrates diagrammatically one particular form of con? :st-ruction, illustrated more clearly in FIGURE 5 and desource. The relative positions between the mouth parts 26, 27, 28 and 2 is ensured by means of membranes 36, .37, 138 which extend across the nozzles 20, 21 and 22 and .are arranged at intervals along the structure, as will be seen from the horizontal section illustrated in FIG- -U RE 4. Within the individual parts the shapes are stabilised by means of cords .39, 40, 41 and 42 as in the previous example.

In this example the air pressure line may be regarded as introducing impedance between the interiors of The correct characteristics are however obtained by virtue of the recuperation regions constituted by the chambers 36, 3'1, 32 and 34. These chambers are made of rigid or relatively rigid material and free communication between the flexible portions of these fixed volume chambers is achieved by making the seatings as large and open as possible. However, it will be appreciated that the effective fixed volumes of chambers 30, 31, 32 and 34 can be varied by making the size of the pressure line 35 larger. If the pressure line 35 is made large enough then the chambers 30, 31, 32 and 34 will become constant pressure chambers as Well as being of fixed volume.

Structures of the kinds shown in FIGURE 2 and in FIGURES 3 and 4 may :be built up in the manner illustrated in FIGURE 5. This figure shows in cross-section a structure according to the invention for example the part 26 in FIGURE 3. It comprises a frame .45 of flexible material such, for example, as foamed rubber or plastic made up in the shape required for the cross-section of the structure. Cords 46 are sewn through this frame so as to lie normal to the axis of the structure. A plurality of frames of this nature are then mounted within a skin 47 of suitable flexible material such as neoprene reinforced with fabric and attached thereto, for example by an adhesive, The feather edge of the structure may be reinforced with a fillet of plastic rubber or like material 48 to which the outer skin may be bonded. Connection to the interior of the structure may be made through a duct 49 which is preferably made as large as possible so as to keep to a minimum the impedance to air flow between the interior of the structure and the recuperation region 30,

In application such as those which have been described above the inherent stiffness of the structure can be moditied for example by the introduction of stiifening ribs, and provision may be made on the outside of the structure for increasing resistance to abrasion. The stiffening ribs can serve also as the abrasion resisting means. Such ribs 50 are shown in FIGURES 5 and 6. The ribs can be of fairly stifi material such as a stiff vulcanised rubber or of a more flexible material. Furthermore, a thin layer 5.1 of abrasion resistant material can be provided be.- tween the ribs 50. The ribs 50, and the layer 51, may be attached to the skin .47 by adhesive and can be attached before or after the skin 47 is attached .to the frames 45, The degree of stiffness introduced by the ribs is likely to depend upon the particular application and care must be taken not to introduce too much stiffness into the structure.

7 illustrates a vehicle which is supported above the surface 61 by a cushion of pressurised gas 6?. The cushion is contained along the sides of the vehicle by side walls 63. At the front and rear of the vehicle the cushion is contained at its upper part by flexible nozzles 6.4 and formed of structures according to the invention,

and for the lower part by curtains of moving air 66,

issuing from the nozzles 64 and 65. The flexible nozzles 64 and 65 are, for example, as the construction illustrated in FIGURE 2. The nozzles are supplied with curtain forming air via ducts .67 from compressors .68. The structures forming the nozzles can be inflated by further compressors 69, which supply air via ducts '70, or the ducts can be connected to the compressors 68. The compressors are driven by engines 71 and air is supplied to the compressors from inlets 72.

The power requirement for the air curtains are only such as will be necessary to form the curtains between the bottoms of the nozzles 64 and 65 and the surface 61, whilst the clearance between the bottom surface 73 of the vehicle 66 and the surface v61 is the distance closed by the curtains of air 66 and also the vertical length of the nozzles. Waves or other obstacles of a height less than the height of the air curtains will pass beneath the nozzles without touching them, but if larger waves or obstacles are met, the nozzles 63 and 64 will readily deflcctupwards.

We claim:

.1. A flexible nozzle formed by a pair of inflatable structures spaced apart from ea h other to define the nozzle passageways theretbetween, each structure being oon strained to assume, when inflated, a predetermined shape and comprising at least two flexible walls bounding at least in part a space of variable volume within said structure adapted to receive an inflating gas, constraining means connecting said flexible walls, means forming a recuperation region of fixed volume associated with said space, means for supplying a gas under pressure to said space to inflate said structure, and means connecting said space and said recuperation region whereby gas may flow between said space and said regionon deformation of said structure to reduce variations in pressure in said space.

2. A flexible nozzle as claimed in claim -1 wherein each space within said structures is in communication with a separate recuperation region.

3. A flexible nozzle arrangement comprising a series of separate spaced apart inflatable structures, each adjacent pair of the inflatable structures defining, at least in part, a nozzle, each of said inflatable structures being constrained to assume, when inflated, a predetermined shape and comprising at least two flexible Walls bounding at least in part a space within said structure adapted to receive an inflating gas, constraining means connecting said flexible walls, means forming a recuperation region associated with said space, means for supplying a gas under pressure to said space to inflate said structure, and means connecting said space and said recuperation region whereby gas may flow between said space and said region on deformation of said structure to reduce variations in pressure in said space.

4. A flexible nozzle arrangement as claimed in claim 3 wherein each space within the said structures is in communication with a separate recuperation region.

5'. A flexible nozzle arrangement as claimed in claim 3 including membranes connecting each adjacent pair of inflatable structures defining a nozzle whereby the portions of said structures forming the nozzle mouths are maintained in predetermined relative positions.

6. A vehicle for travelling over a surface and which in operation is at least partly supported above that surface by a cushion of pressurised gas formed and contained at last in part by a curtain of moving fluid issuing from the lower part of the vehicle, including a flexible nozzle from which at least part of the curtain forming fluid issues, said nozzle being formed by a pair of inflatable structures spaced apart from each other to define the nozzle pasageway therebetween, each structure being constrained to assume, when inflated, a predetermined shape 'and comprising at least two flexible walls bounding at least in part a space of variable volume within said structure adapted to receive an inflating gas, constraining means connecting said flexible Walls, means forming a recuperation region of fixed volume associated with said space, means for supplying a gas under pressure to said space to inflate said structure, and means connecting said space and said recuperation region whereby gas may flow between said space and said region on deformation of said structure to reduce variations in pressure in said space.

7. A vehicle for traveling over a surface and which in operation is at least partly supported above that surface by a cushion of pressurised gas formed and contained at least in part by a curtain of moving fluid issuing from the lower part of the vehicle, including a flexible nozzle arrangement from which at least part of the curtain forming fluid issues, said nozzle arrangement comprising a series of separate spaced apart inflatable structures, each adjacent pair of the inflatable structures defining, at least in part, a nozzle, each of said inflatable structures being constrained toassume, when inflated, a predetermined shape and comprising at least two flexible Walls bounding at least in part .a space Within said structure adapted to receive an inflating gas, constraining means connecting said flexible walls, means forming a recuperation region associated with said space, means for supplying a gas under pressure to said space to inflate said structure, and means connecting said space and said recuperation region whereby gas may flow between said space and said region on deformation of said structure to reduce variations in pressure in said space.

8. A vehicle for traveling over a surface and which in operation is at least partly supported above that surface by a cushion of pressurised gas, comprising side walls extending along the sides and downwardly from the lower part of the vehicle and partially containing said cushion of pressurised gas, a flexible nozzle extending across each end of the vehicle between said side walls and cooperating with said side Walls to contain the upper part of said cushion, and means for causing curtains of moving fluid to issue from said nozzles and contain the lower part of said cushion at the front and rear of the vehicle, each of said nozzles being formed by a pair of inflatable structures spaced apart from each other to define the nozzle passageway therebetween, each structure being constrained to assume, when inflated, a predetermined shape and comprising at least two flexible walls bounding at least in part a space of variable volume within said structure adapted to receive an inflating gas, constraining means connecting said flexible walls, means forming a recuperation region of fixed volume associated with said space, means for supplying a gas under pressure to said space to inflate said structure, and means connecting said space and said recuperation region whereby gas may flow between said space and said region on deformation of said structure to reduce variations in pressure in said space.

9. A flexible nozzle formed by a pair of inflatable structures spaced apart from each other to define the nozzle passageway therebetween, at least one of said structures comprising at least two flexible walls bounding at least in part a space of variable volume within said structure adapted to receive an inflating gas, constraining means connecting said flexible walls, means forming a recuperation region of fixed volume associated with said space, means for supplying a gas under pressure to said space to inflate said structure, and means connecting said space and said recuperation region whereby gas may flow between said space and said region on deformation of said structure to reduce variations in pressure in said space.

10. A flexible nozzle formed by a pair of inflatable structures spaced apart firorn each other to define the nozzle passageway therebetween, at least that one of said structures which constitutes the outer part of the nozzle comprising at least two flexible walls bounding at least in part a space of variable volume within said structure adapted to receive an inflating gas, constraining means connecting said flexible Walls, means forming a recuperation region of fixed volume associated with said space, means for supplying a gas under pressure to said space to inflate said structure, and means connecting said space and said recuperation region whereby gas may flow between said space and said region on deformation of said structure to reduce variations in pressure in said space.

11. A vehicle for traveling over a surface and which in operation is at least partly supported above that surface by a cushion of pressurised gas formed and contained at least in part by a curtain of moving fluid issuing from the lower part of the vehicle, including a flexible nozzle from which at least part of the curtain forming fluid issues, said nozzle being formed by a pair of inflatable structures spaced apart from each to define the nozzle passageway therebetween, at least one of said structures comprising at least two flexible walls bounding at least in part a space of variable volume within said structure adapted to receive an inflating gas, constraining means connecting said flexible walls, means forming a recuperation region of fixed volume in communication with said space, means for supplying a gas under pressure to said space to inflate said structure, and means connecting said space and said recuperation region whereby gas may flow between said space and said region on deformation of said structure to reduce variations in pressure in said space.

References Cited by the Examiner UNITED STATES PATENTS 2,322,790 6/ 1943 Cristadoro 1807 3,077,174 2/ 1963 Cockerell 1807 3,161,247 12/1964 Mackie 1807 FOREIGN PATENTS 1,23 8,499 7/ 1960 France.

OTHER REFERENCES Design News, May 23, 1960, pp. 6 and 7, Ground Cushion Balances with Single Engine. 1

BENJAMIN HERSH, Primary Examiner.

PHILIP ARNOLD, Examiner. 

1. A FLEXIBLE NOZZLE FORMED BY A PAIR OF INFLATABLE STRUCTURES SPACED APART FROM EACH OTHER TO DEFINE THE NOZZLE PASSAGEWAYS THEREBETWEEN, EACH STRUCTURE BEING CONSTRAINED TO ASSUME, WHEN INFLATED, A PREDETERMINED SHAPE AND COMPRISING AT LEAST TWO FLEXIBLE WALLS BOUNDING AT LEAST IN PART A SPACE OF VARIABLE VOLUMNE WITHIN SAID STRUCTURE ADAPTED TO RECEIVE AN INFLATING GAS, CONSTRAINING MEANS CONNECTING SAID FLEXIBLE WALLS, MEANS FORMING A RECUPERATION REGION OF FIXED VOLUME ASSOCIATED WITH SAID SPACE, MEANS FOR SUPPLYING A GAS UNDER PRESSURE TO SAID SPACE TO INFLATE SAID STRUCTURE, AND MEANS CONNECTING SAID SPACE AND SAID RECUPERATION REGION WHEREBY GAS MAY FLOW BETWEEN SAID SPACE AND SAID REGION ON DEFORMATION OF SAID STRUCTURE TO REDUCE VARIATIONS IN PRESSURE IN SAID SPACE. 